LOS ANGELES — Sen. Dianne Feinstein seemed under siege within her own party last fall, with fellow California congressional Democrats openly speculating about possible primary challengers and progressives railing against her brand of centrism.

But with the state’s “top two” primary just four months away, few are talking about her vulnerability anymore.

California’s senior senator is thrashing her main opponent by nearly 30 percentage points in her bid for reelection, according to a poll released late Wednesday. Feinstein posted close to $10 million in cash on hand, while her challenger, state Senate President Pro Tem Kevin de León, reported raising just $500,000 by the end of last year.

His fundraising was so weak — and public polls so lopsided — that organizers of a pro-Feinstein super PAC are quietly considering folding because of the prospect she won’t need outside help.

“In politics, it’s generally a bad idea to say ‘never,’” said Darry Sragow, the longtime Democratic strategist whose California Target Book handicaps races in the state. “But realistically, Kevin is confronted with new fundraising numbers and polling results that often can create a self-fulfilling prophecy.”

Sragow, who managed Feinstein’s 1990 gubernatorial primary campaign in California, said: “The simple matter is, for the most part, contributors like to pick winners. They hate to be on the losing side. And I am very familiar with campaigns that have been on the short end of finance reports and polling, and it was always very tough to get traction.”

De León, a champion of progressive causes in the state Legislature, acknowledged when he entered the race in mid-October that he was facing an uphill climb. But Feinstein’s age — at 84, she is the Senate’s oldest serving member — and centrist politics appeared to offer him an opening.

A Berkeley IGS Poll last year found more than half of California’s registered voters thought it would be a “bad thing” if Feinstein ran for reelection, a number that increased significantly when voters were reminded of Feinstein’s age. Meanwhile, progressive Democrats in California protested outside Feinstein’s home and confronted her at public appearances, criticizing her for her skeptical view of single-payer health care and for her support for some of President Donald Trump’s earliest nominees.

When Feinstein suggested last year that Trump could become a “good president,” De León pounced. Democrats, he said, should “not be complicit in his reckless behavior.”

But now, four months before the state’s June primary election, Feinstein appears immovable, trouncing de León 46 percent to 17 percent among likely California voters, according to a poll by the Public Policy Institute of California.

Though a third of likely voters remain undecided, Feinstein’s advantage among Democrats is even wider than with the electorate overall — 67 percent to 19 percent — a foreboding sign for de León’s bid to run against her from the left. She is polling ahead of de León by wide margins among independent voters, men, women, and people of all racial and ethnic groups.

“It’s just very, very tough for anybody to run against this incumbent senator,” poll director Mark Baldassare said. “We haven’t seen any movement since December, and what a big lift.”

He added, “The numbers speak for themselves.”

First elected in a 1992 special Senate election, Feinstein remains an institution in California politics, and her prospects for reelection appear to have improved since the start of the year. She dodged the specter of a well-funded opponent when billionaire environmentalist Tom Steyer announced in January that he would not challenge Feinstein or run for any other seat this year.

Days later, Feinstein notched a public relations victory when Trump bestowed her with a nickname, “Sneaky Dianne Feinstein,” for releasing congressional testimony on a dossier alleging ties between the Trump campaign and Russia.

For a senator often criticized by progressives for her inclination to compromise, drawing Trump’s ire served to reinforce her anti-Trump credentials.

“Congratulations to California’s own @SenFeinstein on officially earning her own @realDonaldTrump nickname,” House Minority Leader Nancy Pelosi said on Twitter. “Clearly doing something right!”

On Wednesday, Feinstein yoked herself to Pelosi’s hourslong speech on the House floor calling for protections for undocumented immigrants. The senator— who had infuriated activists with her acknowledgment last year that the Deferred Action for Childhood Arrivals program might be on shaky legal ground, said on Twitter, “Leader Nancy Pelosi has held the House floor for 7 hours to advocate for a path to citizenship for Dreamers. Nancy knows how important it is to get this done because California is home to more Dreamers than any other state. Go Nancy!”

In a heavily Democratic state where polls show immigration weighs heavily on voters’ minds, Dan Schnur, a longtime state political analyst, said Feinstein “may need to thank de León by the time the primary’s over.”

“Remember, originally she was fairly noncommittal on whether DACA should be part of the budget agreement,” he said. “Ever since de León first pushed her on DACA, Feinstein has veered hard left, and that’s clearly been to her benefit. A lot of progressive Democrats who may have been nervous by some of her comments last year are being reassured by what she’s said and done since.”

Eddie Kurtz, president and executive director of the liberal advocacy group Courage Campaign, said this week that Feinstein’s rhetoric on DACA and increasingly sharp criticism of Trump are a result of “the pressure that Kevin is putting on her.”

Though Kurtz said Feinstein remains out of step with California’s increasingly liberal politics, “Even if Kevin’s race does nothing other than help her more accurately reflect California through the next election, I think Kevin’s done a great service.”

Kurtz and other critics of Feinstein say it is too soon to write off de León. In California’s primary system, in which the top two vote-getters advance to the November general election regardless of party affiliation, Feinstein and de León both appear likely to advance to a fall runoff.

“I’m not predicting his defeating her,” Kurtz said. “But it just seems like that’s so far away … It would be one thing if it was expected to be over in June. But I feel like it’s way too early to be writing an obituary, since it feels like it’s going to go to November.”

If there is any upside to recent polling for de León, it is that he remains largely unknown to California voters. While Feinstein is seeking a fifth full term in the Senate, nearly two-thirds of likely voters have either never heard of de León or don’t know enough about him to form an opinion, according to the most recent Public Policy Institute of California poll.

Dave Jacobson, a California strategist who helped open a super PAC to support de León, said de León’s fortunes could change at the state Democratic Party convention later this month in San Diego. He said if de León’s supporters can win over enough rank-and-file activists to block Feinstein from receiving the party endorsement — a possibility at conventions that typically attract the party’s most fervent activists — “that symbolically will be a body blow to her campaign.”

Jonathan Underland, a spokesman for de León’s campaign, said Thursday that “the race is going to tighten,” pointing to Feinstein’s support hanging below 50 percent.

“Kevin’s name recognition is low,” he said. “But despite that, Feinstein is still languishing below 50 percent, which is not as strong as one might expect to see for a five-term senator from a blue state.”

Still, de León has never before run for statewide office, and advertising in California’s expensive media markets typically requires millions of dollars that de León has not demonstrated an ability to raise.

Meanwhile, Feinstein, one of the wealthiest members of Congress, loaned her campaign $5 million and reported raising $1 million more from contributors last reporting period. She holds just less than $10 million in cash on hand, a financial advantage so large that sources close to a pro-Feinstein super PAC said organizers of the group are considering closing it.

Sean Clegg, a partner at SCN Strategies, which launched the PAC, said, “We’re going to go forward if there’s a need, and we’re not if there isn’t.”